Rotary drag bits have been used for subterranean drilling for many decades, and various sizes, shapes, and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements. A drag bit can provide an improved rate of penetration (ROP) over a roller cone bit or impregnated diamond drill bit in many formations.
Over the past few decades, rotary drag bit performance has been improved with the use of a polycrystalline diamond compact (PDC) cutting element or cutter, comprised of a planar diamond cutting element or table formed onto a tungsten carbide substrate under high temperature and high pressure conditions. The PDC cutters are formed into a myriad of shapes including, circular, semicircular or tombstone, which are the most commonly used configurations. Typically, the PDC diamond tables are formed so the edges of the table are coplanar with the supporting tungsten carbide substrate or the table may overhang or be undercut slightly, forming a “lip” at the trailing edge of the table in order to improve the wear life of the cutter as it comes into formations being drilled. Bits carrying PDC cutters, which for example, may be brazed into pockets in the bit face, pockets in blades extending from the face, or mounted to studs inserted into the bit body, have proven very effective in achieving high ROP in drilling subterranean formations exhibiting low to medium compressive strengths. The PDC cutters have provided drill bit designers a wide variety of improved cutter deployments and orientations, crown configurations, facilitated optimal nozzle placements and other design alternatives previously not possible with small natural diamond or synthetic diamond cutters. While the PDC cutting element improves drill bit efficiency in drilling many subterranean formations, however, the PDC cutting element is nonetheless prone to wear when operationally exposed to drilling conditions and lessens the life of a rotary bit.
Thermally stable diamond (TSP) is another synthetic diamond, PDC material which can be used as a cutting element or cutter for a rotary drag bit. TSP cutters, which have had catalyst used to promote formation of diamond-to-diamond bonds in the structure removed therefrom, have improved thermal performance over PDC cutters. The high frictional heating associated with hard and abrasive rock drilling applications, creates cutting edge temperatures that exceed the thermal stability of PDC, whereas TSP cutters remains stable at higher operating temperatures. This characteristic also enables them to be furnaced into the face of a matrix-type rotary drag bit.
While the PDC or TSP cutting elements provide better ROP and manifest less wear during drilling as compared to some other cutting element types, it is still desirous to further the life of rotary drag bits and improve cutter life regardless of the cutter type used. Researchers in the industry have long recognized that as the cutting elements wear, i.e., wearflat surfaces develop and are formed on each cutting element coming in contact with the subterranean formation during drilling, the penetration rate (or ROP) decreases. The decrease in the penetration rate is a manifestation that the rotary drag bit is wearing out, particularly when other drilling parameters remain constant. Various drilling parameters include formation type, WOB, cutter position or rake angle, cutter count, cutter density, drilling temperature and RPM, for example, without limitation, and further include other parameters understood by a person of skill in the subterranean drilling art.
While researchers continue to develop and seek out improvements for longer lasting cutters or generalized improvements to cutter performance, they fail to accommodate or implement an engineered approach to achieving longer drag bit life by maintaining or increasing penetration rate or ROP by taking advantage of cutting element wear rates. In this regard, while ROP is many times a key attribute in identifying aspects of the drill bit performance, it would be desirable to utilize or take advantage of the cutting element wear in extending or improving the life of the drag bit.
Accordingly, there is an ongoing desire to improve or extend rotary drag bit life regardless of the subterranean formation type being drilled. There is a further desire to extend the life of a rotary drag bit by beneficially orienting and positioning cutters upon the bit body.